Various composite materials such as fiber-reinforced metals (FRM) produced by reinforcing metals with fibers have recently come into general use for various machine parts and structural members. Although reinforcement fibers for FRM and the like are not easily wet by a matrix metal, especially an aluminum alloy or magnesium alloy, once wet, the reinforcement fibers react with the matrix and undergo degradation. Accordingly, surface treatment is generally applied to the reinforcement fibers, including, for example, a chemical vapor deposition method and a plating method, by which the reinforcement fibers are coated with metals or ceramics in the form of a thin uniform film at the surfaces thereof. However, these methods have various drawbacks. For example, the thin film can peel off due to the difference between the coefficients of thermal expansion for the reinforcement fibers and the matrix, thus reducing the effect of the surface treatment. In addition, if the coating film is made thicker, the reinforcement fibers lose their flexibility, become rigid and brittle, and are easily damaged. Furthermore, a complex apparatus is required for the surface treatment of individual fibers, undesirably increasing the cost of production. Moreover, if FRM is produced by the squeeze casting method, the fiber distribution tends to be uneven and to have coarse and dense portions. This makes it difficult to control the fiber volume fraction (Vf) in FRM. Especially, when the Vf is small, it undesirably restricts the degree of freedom available for the materials design, which is an advantageous feature of FRM or the like containing uniformly dispersed reinforcement fibers.
For overcoming such disadvantages, the combined use of continuous fibers or long fibers with short fibers and/or whiskers as the reinforcement fibers for use in composite materials has been proposed. For example, a method is known using long fibers to form the inside part and short fibers to form the outside part of FRM, as well as a method of preparing a prepreg for FRM by pressure-forming a mixture of long and short fibers.
However, the method requiring separation of long fibers and short fibers in the component complicates the production step for FRM or the like, and the strength of the materials is unsatisfactory. In the method using a mixture of long fibers and short fibers in preparing a prepreg, although short fibers can be applied to the surface of the bundle of long fibers, it is difficult to uniformly deposit the short fibers on the surfaces of the individual long fibers in the inside of the bundle thereby reducing the uniformity of the fibrous material obtained.
For overcoming the problems described above, the present inventors have previously proposed a method of depositing short fibers, whiskers or powders to the surface of individual fibers by dipping a bundle of continuous fibers into a liquid containing the short fibers, whiskers or powders suspended therein (U.S. patent application Ser. No. 865,293, now U.S. Pat. No. 4,732,779). Although this method is excellent for the preparation of FRM, it has been found as the result of further study that the method is not always completely successful, depending on conditions such as the composition of the matrix in the FRM. For example, when using ordinary continuous fibers, a method is still sought for obtaining satisfactory strength in the axial direction of the fiber in FRM materials, fatigue strength in FRP materials, and heat resistance in FRC materials.
The problem remains that the continuous fibers are not uniformly dispersed to an extent sufficient for practical use, and the volume fraction of the fibers cannot be controlled over a wide range, preventing a satisfactory improvement in mechanical properties such as strength.